The quality of superconductor thin films is described by several parameters: critical temperature, T.sub.c, surface resistance, R.sub.s, critical current density, J.sub.c, critical magnetic field, H.sub.c, etc. For microwave and millimeter wave applications, the most important parameter of a superconductor film is the surface resistance, R.sub.s, at a given frequency, measured as a function of temperature, current density (or rf magnetic field). Measurement of these parameters accurately is not only necessary for superconductor material research and applications, but is also important for controlling the quality of manufacturing superconducting film.
One method for measuring surface resistance is called "TE.sub.011 mode cavity end wall replacement", Muller et al., J. Superconductivity, Vol. 3, p. 235-242 (1990). It utilizes a copper cylindrical cavity operating at TE.sub.011 mode with one of its two end walls replaced by a superconductor film. The R.sub.s of the film can be determined by comparing the Q-values of the cavity with the sample to the same cavity with a calibration standard film (such as niobium or copper) having a known R.sub.s value. This method has the following shortcomings: 1) it requires calibration, so it is not an absolute measurement; 2) the accuracy is limited by the fact that the R.sub.s of the sample film under test only contributes a small portion of the loss in the cavity; 3) the measurable range of the R.sub.s is limited at the low end by the poor sensitivity of this method.
Another method for measuring R.sub.s is called "parallel plate resonator" as disclosed in Taber, R., Rev. Sci. Instrum., Vol. 61, p. 2200-2206 (1990). It is constructed by two pieces of superconducting film separated by a thin dielectric spacer. The R.sub.s of the superconductor film can be determined by measuring the Q-value of the resonator. This method has the following shortcomings: 1) because the spacer is very thin, it is very difficult to couple the rf power in and out of the resonator; 2) since the Q-value is relatively low, the measurable range of R.sub.s is limited at the high end by a weak coupling; 3) since the parallel plate resonator is an open structure, the rf magnetic field is not confined, which results in poor accuracy and case mode interference; 4) it is not an absolute method, calibration is required.
Yet another method is called "dielectric resonator" as disclosed by Fiedziusko et al., IEEE-MTT-S International Microwave Symposium Digest, Vol. 2, p. 555-558 (1989) and by Llopis et al., J. Less-Common Metals, Vols. 164, 165, p. 1248-1251 (1990). There are two different versions. One version involves putting a dielectric resonator on top of the sample superconducting film under testing. Again the R.sub.s of the film can be determined by measuring the Q-value of the resonator. This method has the following shortcomings: 1) the open structure makes it difficult to confine the rf fields, which results in poor accuracy and moding problems; 2) means for holding the dielectric resonator in the right place is a problem. The second version is a superconductor-dielectric-superconductor sandwich. Adding the second superconductor film solved the problems encountered in use of the first version. However, since the dielectric material used has a poor loss tangent factor, the sensitivity of this method is limited.
Currently available apparatus for measuring surface resistance are not suitable for use as a production tool because of their limitations. Films cannot be tested at high power. Measurement is not accurate, and its reproducibility is poor. The dynamic range is limited. The assembly is time consuming. Finally, the measurement is very sensitive to how the films are assembled in the apparatus. Thus there is a need for an apparatus suitable for use in quality control operations for monitoring superconducting film manufacturing processes.
The present invention provides apparatus suitable for improved characterization of high temperature superconducting thin films. Major improvement in performance has been achieved as well as the ability to use the same concept design to make the resonators that can characterize different sizes of superconducting thin films.